The successful Soviet effort to return soil samples from the Moon with robotic spacecraft at the beginning of the 1970s inspired the nation's engineers to take on the much bigger challenge of getting a piece of Mars. According to the 5NM concept, a giant N1 or N-1M rocket would launch a 98-ton behemoth in September 1975 on a round trip to the Red Planet, resulting in the delivery of a soil sample from the mysterious Red Planet.

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The earliest Soviet plans for delivering Martian soil naturally grew out of similar missions to the Moon developed in the second half of the 1960s at the NPO Lavochkin design bureau led by Georgy Babakin. At the beginning of 1970, soon after the lunar sample return missions had reached the launch pad, Babakin directed his engineers to prepare a technical proposal for a Martian sample return mission. By the summer of the same year, the team had already formulated a concept of the spacecraft designated 5NM. According to the plan, the yet-to-be-operational N1 or N-1M super-heavy rocket would launch a 98-ton behemoth into low-Earth orbit in September 1975. From there, the two-stage booster would accelerate a 20-ton spacecraft on its way to Mars.

The spacecraft itself included a 3,600-kilogram Earth-to-Mars cruise stage built around a thorus-shaped instrument module inherited from the Mars-71 project and a spherical propellant tank originally developed for the Mars-69 vehicle.

The cruise stage had its own propulsion system to perform trajectory corrections on the way to Mars. The vehicle would put itself on a Mars flyby trajectory to serve as a communications relay station between the lander and ground control.

The second module of the spacecraft was the 16-ton lander equipped with a deployable aerodynamic brake consisting of 30 petals attached to a 6.5-meter central cone. After the spacecraft had entered an interplanetary trajectory, the petals of the heat shield would be deployed, forming an asymmetrical aero-shell structure with a diameter of 11 meters, which would help the lander to glide while slowing down in the weak Martian atmosphere. The flight control system responsible for the landing was housed in the instrument module behind the aero-shell. The instruments included a velocity measuring system using the doppler principle, an altimeter, radio and power-supply systems.

When the descending lander slowed down to 200 meters per second, the heat shield would be discarded. The final descent to the Martian surface would be conducted with the help of four variable-thrust rocket engines fed from four spherical tanks.

Once on the surface, the mission control would command the lander via a decimeter-frequency communication system to conduct panoramic imaging of the surrounding landscape and zero-in on objects, which would look most promising to the scientists. The departure from Mars was planned around three days after the arrival, as the spacecraft autonomously established its precise location on the Martian surface.

On top of the lander sat the two-stage Earth-return rocket, which included a 750-kilogram Mars-to-Earth cruise stage, whose design was based on the Venera-4 and Venera-6 spacecraft. It was topped with the 15-kilogram landing capsule, which could carry 200 grams of Martian soil back to Earth.

The Earth-return rocket would first enter a 500-kilometer Martian orbit with a period of 12 hours, where it would remain for 10 months, waiting for a favorable mutual position between Earth and Mars.

Upon approaching Earth, the landing capsule would separate from the cruise stage and descent through the atmosphere. When it had slowed down to 200 meters per second, the capsule would release its parachute. After touchdown, the capsule would deploy a radar beacon to help the search teams.

Similarly to the Soviet lunar exploration program, the Martian sample return mission would be preceded to the Red Planet by a rover, scheduled for launch in 1973. It would be delivered to Mars with the 4NM spacecraft, which would test key technologies of the Mars landing, most importantly, the deployable gliding aero-shell. (633)